This invention relates to a self-oscillating modulator, comprising a comparator and a power amplification stage for pulse modulation, and a higher order oscillating loop comprising first feedback means and first forward means securing stable oscillating conditions, wherein said first feedback means is arranged to supply a first feedback signal to said first forward means, which is arranged to provide a modulating signal to said comparator.
The invention also relates to a switching power conversion system, such as DC-AC (e.g. audio amplification), DC-DC or AC-AC conversion systems using such a modulator. The invention may advantageously be used for improved power conversion in any system, in particular precision DC-AC conversion systems as high efficiency audio amplification.
The pulse modulator is a central element of any power conversion system. Most switching power converters are based on Pulse Width Modulation (PWM) as a means to control efficient conversion between domains (DC or AC).
A typical power converter includes a PWM modulator, a switching power conversion stage, a filter and a control system. A prior art system of this type is described in U.S. Pat. No. 4,724,396 (1988) and by Mr. Attwood in Journal of the AES, November 1983. p. 842-853. However, PWM has a range of shortcomings also well known to the art, mainly due to the implementation of the carrier generation. This limits the system bandwidth and complicates design. Also, a stable and robust control system design is difficult.
In order to overcome these drawbacks, a controlled self-oscillating modulator (COM) was introduced in the applicant""s international patent application PCT/DK97/00497. The disclosed modulator eliminates the need of a carrier generator, with a range of advantages, described in detail in said document.
A problem with this technique is that voluminous and expensive power components are needed, in order to compensate for filter inductor currents. Further, problems may occur when the modulator is connected to a reactive load or open load, requiring e.g. RC-Zobel compensation. The COM technology is also difficult to implement in systems with greater bandwidth. In short, it could be said that even though the COM approach overcomes some of the disadvantages of the conventional PWM technology, some limitations are still present.
Accordingly, a primary object of the invention has been to provide a superior modulation technique in switching power conversion systems that overcomes fundamental problems related to conventional techniques.
A second object has been to provide a pulse modulation in general power conversion systems that provides significant advantages in performance, topological simplification, improved robustness and stability compared to prior art.
These objects are achieved by a controlled self-oscillating modulator (COM) of the type mentioned above, characterized by current measurement means, arranged to measure a value of a current supplied by said power amplification stage to a load connected to the modulator, said measured value being supplied to said first feedback means, wherein said first feedback means has a transfer function adapted to generate said first feedback signal based on said measured value, and said first forward means has a transfer function adapted to generate said modulating signal based on said first feedback means and an input signal.
By measuring the current that is supplied to the load, the load control is much improved, eliminating the need of further load compensation. Also, any filter inductor currents are compensated, as the modulator acts as a powerful, wideband control system, leading to lower requirements on the power components.
As the current measurement itself implements a first order transfer function, and thus introduces a first pole in the oscillating loop, this leads to a simplification of the design. Especially in applications where miniaturization is important, such as mobile, personal audio equipment, this is an important advantage over prior art.
The COM according to the invention thus aims to combine the advantages of the COM technology with the advantages of current controlled PWM technology, resulting in a system presenting the following features:
The system is inherently unstable
No carrier generator is needed, saving components.
The power supply variable VS is eliminated from the effective loop transfer function.
The system acts as a wideband control system.
Clean modulation (no feedback noise and poor carrier signal).
A controllable variable switching frequency for improved efficiency and EMI.
This makes the modulator according to the invention very suitable in all types of precision DC-AC conversion applications, such as audio applications.
The modulator preferably includes means for feedback of an output voltage from said amplification stage. This enables a very compact design combining modulation and control system surrounding the central power conversion stage. Whereas combined voltage and current feedback PWM systems are well known in the art, said COM based two loop topology is an advantageous improvement and new to the art.
According to an embodiment of the voltage feedback, the first feedback means is connected to said output voltage, and the first feedback means is adapted to generate said first feedback signal based on said current value and said voltage. This can be an advantageous way to implement the double feedback, reducing the number of components.
According to another embodiment of the voltage feedback, the modulator comprises a second feedback means connected to said output voltage, and adapted to generate a second feedback signal, and a second forward means, adapted to supply said first forward block with said input signal based on said second feedback signal and a second signal. Although this implementation is somewhat more complex, it can be advantageous in some applications, where it is desirable to let the current measurement and the voltage feedback with different transfer functions.
The power amplification stage can comprise an output filter, and the second feedback means can then be connected to an output from said output filter. This permits a first filtering of the voltage before it is fed back in the feedback path.
In a preferred embodiment, the means for current measurement is adapted to implement protection functions as limitations of peak current and long term current.
The controlled oscillation modulator can advantageously be used in precision voltage or current controlled DC-AC conversion as e.g. power amplifiers for audio use.